Orange Luminescence and Structural Properties of Three Isostructural Halocyclohexylisonitrilegold(I) Complexes

Article abstract of DOI:10.1021/ic0341946

The preparation of three isonitrile complexes (CyNC)Au^ICl, (CyNC)Au^IBr and (CyNC)Au^Il, along with their structural and spectral characterization, are reported. X-ray crystal structures reveal that these crystallize in the same space group and have closely related structures. The structures involve pleated chains of linear, two-coordinate monomers that are arranged in a head-tail fashion. However, these chains vary significantly in the degree of aurophilic interactions among the individual molecules. Thus, (CyNC)Au^ICl forms infinite chains with alternating Au...Au distances of 3.3894(7) and 3.5816(7) A. Within the chains of (CyNC)Au^IBr, however, the alternation of Au...Au distances is more pronounced so that there are dimers, with an Au...Au distance of 3.4864(9) A, and neighboring gold centers at 3.7036(9) A. In (CyNC)Au ^Il, the gold-gold contacts do not lie within the range of significant aurophilic bonding. The closest Au...Au distance is 3.7182(11) A while every other Au...Au distance is 3.9304(12) A. The steric factor of the X ligand and dipole-dipole interactions between the antiparallel complexes is much more significant than aurophilic interactions in governing the self-association of the complexes in this series. The colorless crystals of each solid display an orange luminescence band with a strikingly large Stokes' shift (～21 000 cm^-1, 2.6 eV). However, considerable care had to be taken to ensure that the crystals used for the study of the luminescence were free of a surface impurity that produced a turquoise-green luminescence in (CyNC)Au^ICl. The diffuse reflectance spectra for the solids show a similar three-band pattern in the 200-330 nm range.

Full text of DOI:10.1021/ic0341946

Inorg. Chem. 2003, 42, 6741−6748
Orange Luminescence and Structural Properties of Three Isostructural
Halocyclohexylisonitrilegold(I) Complexes
Rochelle L. White-Morris, Marilyn M. Olmstead, and Alan L. Balch*
Department of Chemistry, UniVersity of California, DaVis, California 95616
O. Elbjeirami and Mohammad A. Omary*
Department of Chemistry, UniVersity of North Texas, Denton, Texas 76203
Received February 20, 2003
I
I
I
The preparation of three isonitrile complexes (CyNC)Au Cl, (CyNC)AuBr, and (CyNC)Au I, along with their structural
and spectral characterization, are reported. X-ray crystal structures reveal that these crystallize in the same space
group and have closely related structures. The structures involve pleated chains of linear, two-coordinate monomers
that are arranged in a head−tail fashion. However, these chains vary significantly in the degree of aurophilic
I
interactions among the individual molecules. Thus, (CyNC)AuCl forms infinite chains with alternating Au‚‚‚Au distances
I
of 3.3894(7) and 3.5816(7) Å. Within the chains of (CyNC)AuBr, however, the alternation of Au‚‚‚Au distances is
more pronounced so that there are dimers, with an Au‚‚‚Au distance of 3.4864(9) Å, and neighboring gold centers
I
at 3.7036(9) Å. In (CyNC)AuI, the gold−gold contacts do not lie within the range of significant aurophilic bonding.
The closest Au‚‚‚Au distance is 3.7182(11) Å while every other Au‚‚‚Au distance is 3.9304(12) Å. The steric factor
of the X ligand and dipole−dipole interactions between the antiparallel complexes is much more significant than
aurophilic interactions in governing the self-association of the complexes in this series. The colorless crystals of
each solid display an orange luminescence band with a strikingly large Stokes’ shift (∼21 000 cm-1, 2.6 eV).
However, considerable care had to be taken to ensure that the crystals used for the study of the luminescence
I
were free of a surface impurity that produced a turquoise-green luminescence in (CyNC)Au Cl. The diffuse reflectance
spectra for the solids show a similar three-band pattern in the 200−330 nm range.
Introduction
orange, luminescent form (λmax ) 631 nm) when exposed
to vapors of organic solvents. The trimer, {Au3(MeNd
Although the majority of two-coordinate gold(I) complexes
are colorless, many are luminescent in the visible region.1
The luminescence from such gold(I) complexes has attracted
considerable attention because of the variety of factors that
can influence the luminescence and the range of novel optical
phenomena that have been observed, particularly for crystal-
line solids. For example, the binuclear dithiocarbamate
complex, AuI2{S2CN(n-pentyl)2}2, studied by Eisenberg,
Gysling, and co-workers crystallizes in either colorless or
orange forms.2 The colorless form is transformed into the
COMe)3}, displays solvoluminescence emission at λmax
)
552 nm that is triggered by contact of previously photoir-
radiated crystals with solvents.3,4 Crystalline [(CyNC)2AuI]-
(PF6) forms yellow and colorless polymorphs that display
distinct luminescence, λmax ) 424 nm (colorless) or 480 nm
(yellow), at 298 K.5 Fackler and co-workers showed that
colorlesscrystalsof[(1,3,5-triaza-7-phosphaadamantane)2Au]-
[Au(CN)2] are nonluminescent but become photoluminescent
after grinding.6 In these examples, the observation of
(3) Vickery, J. C.; Olmstead, M. M.; Fung, E. Y.; Balch, A. L. Angew.
Chem., Int. Ed. Engl. 1997, 36, 1179.
(4) Fung, E. Y.; Olmstead, M. M.; Vickery, J. C.; Balch, A. L. Coord.
Chem. ReV. 1998, 171, 151.
(5) White-Morris, R. L.; Olmstead, M. M.; Balch, A. L. J. Am. Chem.
Soc. 2003, 125, 1033.
(6) Assefa, Z.; Omary, M. A.; McBurnett, B. G.; Mohamed, A. A.;
Patterson, H. H.; Staples, R. J.; Fackler, J. P., Jr. Inorg. Chem. 2002,
41, 6274.
* Authors to whom correspondence should be addressed. E-mail:
omary@unt.edu (M.A.O.); albalch@ucdavis.edu (A.L.B.).
(1) For reviews, see: (a) Forward, J. M.; Fackler, J. P., Jr.; Assefa, Z. In
Optoelectronic Properties of Inorganic Compounds; Roundhill, D. M.,
Fackler, J. P., Jr., Eds.; Plenum Press: New York, 1999; p 195. (b)
Yam, V. W. W.; Lo, K. K. W. Chem. Soc. ReV. 1999, 28, 323.
(2) Mansour, M. A.; Connick, W. B.; Lachicotte, R. J.; Gysling, H. J.;
Eisenberg, R. J. Am. Chem. Soc. 1998, 120, 1329.
10.1021/ic0341946 CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/19/2003
Inorganic Chemistry, Vol. 42, No. 21, 2003 6741

White-Morris et al.
luminescence has been tied to the presence of close Au‚‚‚Au
contacts within the solids. Attractive aurophilic interactions
(aurophilic bonding) between closed shell gold(I) centers are
generally acknowledged to exist in solids whenever adjacent
Au‚‚‚Au contacts are less than ca. 3.6 Å.7,8 Such interactions
represent a significant factor in determining the solid-state
organization of many gold(I) complexes.9,10 Correlation
effects strengthened by relativistic effects, as suggested by
Pyykko¨,11,12 and/or hybridization of the 6s and 6p orbitals
with the 5d orbitals, as suggested by Hoffmann,13 are
responsible for the aurophilic bonding. Experimental studies
that examined the rotational barriers in binuclear Au(I)
complexes have shown that the strength of the attractive
aurophilic interaction is comparable to hydrogen bonding,
ca. 7-11 kcal/mol.14,15
Aggregation via aurophilic bonding can have a profound
influence on the emission of gold(I) complexes. For example,
Patterson and co-workers have shown that the simple
[Au(CN)2]- and [Ag(CN)2]- ions aggregate under a range
of conditions and that the aggregated forms show remarkable
variations in their luminescence.16-18 Thus, samples of KCl
doped with varying amounts of K[Au(CN)2] show multiple
emissions whose relative intensities depend on the dopant
level, temperature, and excitation wavelength.19 Similarly,
the luminescence from solutions of K[Au(CN)2] can be
“tuned” to occur from 275 to 470 nm depending upon the
concentration and solvent.20 Related studies have shown that
the colorless gold carbene cation [Au{C(NHMe)2}2]+ ag-
gregates in different fashions in salts with different anions
(e.g., (PF6)-, (BF4)-, Cl-, Br-) and that each salt shows its
own unique luminescence.21,22
The neutral isonitrile compounds, (RNC)AuIX, present a
diverse array of supramolecular structures. Variation of the
R group produces aggregates that include dimers, one-
dimensional extended-chain polymers, and two-dimensional
polymeric sheets.23-31 Despite the simplicity in their molec-
ular structure, spectroscopic investigations of the (RNC)-
AuIX family of compounds are very limited.23,29 However,
a photophysical/photochemical study of the closely related
(OC)AuICl showed that it is luminescent with emission at
663 nm from the solid, while the absorption in solution
occurs at ca. 250 nm.32
A meaningful comparison of intermolecular d10-d10
interactions is facilitated through examination of structurally
and compositionally similar complexes that crystallize in an
isostructural fashion.33 Thus, by studying isostructural Au(I)
and Ag(I) compounds, Schmidbaur and co-workers demon-
strated that gold is smaller than silver, contrary to what was
believed prior to these studies.34 In another recent compara-
tive study of three complexes that crystallize in the same
space group, Fackler and co-workers have shown that the
intramolecular Ag-Au bonding in a binuclear organosulfur
complex was stronger than the Au-Au and Ag-Ag bonding
in the corresponding homonuclear complexes.35
Here, we report the structural and spectral characterization
of three newly prepared isonitrile complexes (CyNC)AuICl,
(CyNC)AuIBr, and (CyNC)AuII in order to examine the
effects of the different halide ligands on the self-association
and the luminescence. A complementary study of a series
of complexes of the type (RNC)AuICN in which the R group
is varied while keeping X ) CN has been reported
elsewhere.36
Results
(7) Schmidbaur, H. Gold: Progress in Chemistry, Biochemistry and
Technology; Wiley: New York, 1999.
Colorless crystals of (CyNC)AuICl and (CyNC)AuIBr were
obtained by treating aqueous solutions of HAuCl4‚H2O or
HAuBr4‚H2O with cyclohexyl isonitrile, which acts both as
(8) Grohmann, A.; Schmidbaur, H. In ComprehensiVe Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;
Elsevier: Oxford, 1995; Vol. 3, p 1.
(9) Jones, P. G. Gold Bull. 1986, 19, 46; 1983, 16, 114; 1981, 14, 159;
1981, 14, 102.
(10) Pathaneni, S. S.; Desiraju, G. R. J. Chem. Soc., Dalton Trans. 1993,
319.
(11) (a) Pyykko¨, P.; Li, J.; Runeberg, N. Chem. Phys. Lett. 1994, 218, 133.
(b) Pyykko¨, P.; Mendizabal, F. Chem.-Eur. J. 1997, 3, 1458. (c)
Pyykko¨, P.; Runeberg, N.; Mendizabal F. Chem.-Eur. J. 1997, 3,
1451.
(12) For a review see: Pyykko¨, P. Chem. ReV. 1997, 97, 597.
(13) (a) Merz, K. M., Jr.; Hoffmann, R. Inorg. Chem. 1988, 27, 2120. (b)
Jiang, Y.; Alvarez, S.; Hoffmann, R. Inorg. Chem. 1985, 24, 749. (c)
Mehrotra, P. K.; Hoffmann, R. Inorg. Chem. 1978, 17, 2187. (d)
Dedieu, A.; Hoffmann, R. J. Am. Chem. Soc. 1978, 100, 2074.
(14) Schmidbaur, H.; Graf, W.; Mu¨ller, G. Angew. Chem., Int. Ed. 1988,
27, 417.
(15) Harwell, D. E.; Mortimer, M. D.; Knobler, C. B.; Anet, F. A. L.;
Hawthorne, M. F. J. Am. Chem. Soc. 1996, 118, 2679.
(16) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H. J. Am.
Chem. Soc. 2000, 122, 10371.
(23) Ecken, H.; Olmstead, M. M.; Noll, B. C.; Attar, S.; Schlyer, B.; Balch,
A. L. J. Chem. Soc., Dalton Trans. 1998, 3715.
(24) Schneider, W.; Angermaier, K.; Sladek, A.; Schmidbaur, H. Z.
Naturforsch. B 1996, 51, 790.
(25) Bonati, F.; Minghetti, G. Gazz. Chim. Ital. 1973, 103, 373.
(26) Eggleston, D. S.; Chodosh, D. F.; Webb, R. L.; Davis, L. L. Acta
Crystallogr. 1986, C42, 36.
(27) Lentz, D.; Willemsen, S. J. Organomet. Chem. 2000, 612, 96.
(28) Mathieson, T. J.; Langdon, A. G.; Milestone, N. B.; Nicholson, B. K.
J. Chem. Soc., Dalton Trans. 1999, 201.
(29) Perreault, D.; Drouin, M.; Michel, A.; Harvey, P. D. Inorg. Chem.
1991, 30, 2.
(30) Liau, R.-Y.; Mathieson, T.; Schier, A.; Berger, R. J.; Runeberg, N.;
Schmidbaur, H. Z. Naturforsch. 2002, 57b, 881.
(31) Jia, G.; Puddephatt, R. J.; Vittal, J. J.; Payne, N. C. Organometallics
1993, 12, 263. (b) Jia, G.; Payne, N. C.; Vittal, J. J.; Puddephatt, R.
J. Organometallics 1993, 12, 4771.
(17) Rawashdeh-Omary, M. A.; Omary, M. A.; Shankle, G. E.; Patterson,
H. H. J. Phys. Chem. B 2000, 104, 6143.
(18) Omary, M. A.; Patterson, H. H. J. Am. Chem. Soc. 1998, 120, 7696.
(19) Hettiarachchi, A. R.; Rawashdeh-Omary, M. A.; Kanan, S. M.; Omary,
M. A.; Patterson, H. H.; Tripp, C. P. J. Phys. Chem. B 2002, 106,
10058.
(20) Rawashdeh-Omary, M. A.; Omary, M. A.; Patterson, H. H.; Fackler,
J. P., Jr. J. Am. Chem. Soc. 2001, 123, 11237.
(21) White-Morris, R. L.; Olmstead, M. M.; Jiang, F.; Tinti, D. S.; Balch,
A. L. J. Am. Chem. Soc. 2002, 124, 2327.
(32) Vogler, A.; Kunkely, H. J. Organomet. Chem. 1997, 541, 177.
(33) We use the term “isostructural” for crystals of different compounds
that exist with the same space group, with similar cell dimensions,
and similar, but not necessarily identical, molecular geometries and
supramolecular organizations.
(34) (a) Tripathi, U. M.; Bauer, A.; Schmidbaur, H. J. Chem. Soc., Dalton
Trans. 1997, 2865. (b) Bayler, A.; Schier, A.; Bowmaker, G. A.;
Schmidbaur, H. J. Am. Chem. Soc. 1996, 118, 7006.
(35) Rawashdeh-Omary, M. A.; Omary, M. A.; Fackler, J. P., Jr. Inorg.
Chim. Acta 2002, 334, 376.
(22) White-Morris, R. L.; Olmstead, M. M.; Jiang, F.; Balch, A. L. Inorg.
Chem. 2002, 41, 2313.
(36) White-Morris, R. L.; Stender, M.; Tinti, D. S.; Balch, A. L.; Rios, D.;
Attar, S. Inorg. Chem. 2003, 42, 3237.
6742 Inorganic Chemistry, Vol. 42, No. 21, 2003

Orange Luminescence and Structure of Isonitrile AuI
Table 1. Crystallographic Data
(CyNC)AuICl
(CyNC)AuIBr
(CyNC)AuII
empirical formula C7H11AuClN
C7H11AuBrN
386.04
C7H11AuIN
433.03
fw
341.58
color, habit
cryst syst
space group
a/Å
b/Å
c/Å
colorless, plate colorless, needle colorless, plate
monoclinic
P21/n
monoclinic
P21/n
monoclinic
P21/n
6.065(2)
21.608(8)
6.614(2)
96.53(2)
861.2(5)
4
6.1467(15)
21.598(5)
6.7740(15)
96.756(16)
893.1(4)
4
6.3222(19)
21.610(5)
7.1266(18)
97.439(7)
965.4(4)
4
â/deg
V/Å3
Z
T/K
91(2)
2.635
17.318
4.4
9.8
91(2)
2.871
20.875
3.2
9.3
91(2)
2.979
18.369
6.8
17.4
d/g cm-1
µ/mm-1
R1a(obsd data)
wR2b
a R1 ) Σ ||Fo| - Fc|/Σ Fo|. b wR2 ) [Σ[w(Fo - Fc )2]/Σ[w(Fo )2]]1/2
.
2
2
2
Figure 1. The structural organization of (CyNC)AuICl with 50% thermal
contours.
Table 2. Selected Bond Lengths and Angles
(CyNC)AuICl
(CyNC)AuIBr
(CyNC)AuII
Distances, Å
Au-C
1.961(14)
2.258(4)
3.3879(16)
3.5875(16)
1.972(7)
1.953(14)
2.5319(10)
3.7182(11)
3.9304(12)
Au-X
2.3728(8)
3.4864(9)
3.7036(9)
Au(1)-Au(1)′
Au(1)-Au(1)′′
Angles, deg
X-Au-C
Au-C-N
Au-Au-Au
178.2(4)
178.9(13)
142.92(4)
178.05(17)
178.8(6)
140.81(2)
178.5(3)
178.4(11)
137.40(3)
Symmetry Code
′
′′
-x, 2-y, -z
-x, 2-y, 1-z
2-x, -y, 2-z
2-x, -y, 1-z
1-x, -y, 1-z
1-x, -y, 2-z
a reducing agent and as a ligand. (CyNC)AuICl was also
prepared by reaction of (Me2S)AuICl or (tetrahydrothio-
phene)AuICl with cyclohexyl isonitrile. Colorless (CyNC)-
AuII was obtained from (CyNC)AuICl by metathesis with
sodium iodide. Crystallization procedures are given in the
Experimental Section.
Figure 2. The structural organization of (CyNC)AuIBr with 50% thermal
contours.
Crystallographic Studies. The crystal data are given in
Table 1. Table 2 contains selected interatomic distances and
angles for each of the three neutral complexes. Figures 1, 2,
and 3 present diagrams showing the individual molecules
and the intermolecular interactions.
Crystals of (CyNC)AuICl, (CyNC)AuIBr, and (CyNC)-
AuII are isostructural. The unit cell volume as well as the
lengths of the a and c axes expand in the order Cl < Br <
I, as expected from the increasing size of the halide ligand.
One molecule is present in each asymmetric unit. Individual
molecules in the three compounds differ primarily in the
Au-X distances as can be seen from the data in Table 2.
Each gold atom is nearly linearly coordinated by the two
ligands.
Crystals of (CyNC)AuICl grow in two different morphol-
ogies, plates, which tend to have rather irregular shapes, and
blocks. However, crystallographic data collected on several
crystals of both morphologies produced the same structural
results.
In (CyNC)AuICl, the C-Au distance is 1.961(4) Å and
the Au-Cl distance is 2.258(4) Å. For comparison, the
C-Au and Au-Cl distances in (t-BuNC)AuICl are 1.92(1)
and 2.249(3) Å, respectively.26 In (CyNC)AuICl, the cyclo-
hexyl groups are in their normal chair conformations with
the isonitrile groups in equatorial positions. Individual
molecules of (CyNC)AuICl aggregate through aurophilic
bonding to form infinite chains with alternating Au‚‚‚Au
distances of 3.3879(16) and 3.5875(16) Å. The chain is
pleated with an Au‚‚‚Au‚‚‚Au angle of 142.92(4)°.
In the bromo complex, the C-Au and Au-Br distances
are 1.972(7) and 2.3728(8) Å while the C-Au-Br angle is
178.05(17)°. These distances are consistent with those in (t-
BuNC)AuIBr, where the C-Au and Au-Br distances are
1.939(8) and 2.370(1) Å and the C-Au-Br angle is
177.5(8)°.24 Molecules of (CyNC)AuIBr weakly self-associ-
ate to form dimers with an Au‚‚‚Au distance of 3.4864(9)
Å. These dimers are situated so that they form extended loose
chains with neighboring Au‚‚‚Au distances of 3.7036(9) Å.
While this structure is isostructural with the chloride
analogue, the two different aurophilic contacts are roughly
0.10 Å longer in the bromide compound. The Au‚‚‚Au‚‚‚
Au angle is 140.81(2)°.
In the iodo complex, the C-Au distance is 1.953(14) Å
and the Au-I distance is 2.5319(10) Å while the C-Au-I
Inorganic Chemistry, Vol. 42, No. 21, 2003 6743

White-Morris et al.
Figure 3. A stereoscopic view of the structure of (CyNC)AuII. The shortest Au‚‚‚Au contacts are 3.7182(11) and 3.9304(12) Å.
Figure 4. Photographs of the luminescence from two crystals of (CyNC)AuICl at ambient temperature. Crystal I (left) shows a uniform orange luminescence
upon exciting with 254 nm while crystal II (right) also shows additional turquoise-green luminescence from small portions of the crystal upon exciting with
365 nm. The pictures were taken with a Nikon optical microscope (ECLIPSE ME600L) interfaced with a computer-controlled Nikon digital still camera
(DXM12000). Excitations were by means of a hand-held dual-wavelength mercury UV lamp.
angle is 178.5(3)°. For comparison, the C-Au and Au-I
distances in (t-BuNC)AuII are 1.95(1) and 2.513(1) Å and
the C-Au-I angle is 177(1)°.30 Although crystals of
(CyNC)AuII are isostructural with the chloro and bromo
analogues, the structure does not contain any gold-gold
contacts that are indicative of significant aurophilic bonding.
Within the pleated chains of individual (CyNC)AuII mol-
ecules, the shortest Au‚‚‚Au distance is 3.7182(11) Å and
the other Au‚‚‚Au distance is 3.9304(12) Å. The Au‚‚‚Au‚
‚‚Au angle is 137.40(3)°.
and 2249 cm-1 for (CyNC)AuII. Since these ν(CN) values
are all larger than that of the free ligand (ν(CN) ) 2142
cm-1), the isonitrile acts as a strong σ-donor and a weak
π-acceptor in this family of complexes.
Crystals of (CyNC)AuICl, (CyNC)AuIBr, and (CyNC)-
AuII display an orange luminescence, the intensity of which
is enhanced by lowering the temperature. Figure 4 shows
photographs taken under an optical microscope of the
luminescence from two crystals of (CyNC)AuICl at ambient
temperature. Crystal I, which is a block, shows the uniform
orange luminescence under irradiation at λ < 280 nm. Such
luminescence is characteristic of a sample that is pure at the
single crystal level. Plates of this material can also show
this uniform orange luminescence. Crystal II, which is an
Spectroscopic Studies. Infrared spectral data for the three
complexes are given in the Experimental Section. These
spectra are all similar with small differences in the ν(CN):
2256 cm-1 for (CyNC)AuICl, 2254 cm-1 for (CyNC)AuIBr,
6744 Inorganic Chemistry, Vol. 42, No. 21, 2003

Orange Luminescence and Structure of Isonitrile AuI
irregular plate, shows a nonuniform luminescence that
includes portions with a turquoise-green-emitting material
that coats some areas of the crystal. While crystal II was
taken from a sample that appears pure, it clearly contains
an impurity (as yet unidentified) that is apparent only under
microscopic examination with UV light with λ in the range
of ca. 320-370 nm. The impurity in crystal II is not
discernible under the microscope with the transmitted light.
However, with irradiation at λ < 280 nm, the plate also
produces the orange luminescence characteristic of (CyNC)-
AuICl in all parts of the crystal. Crystals of both types were
indistinguishable by single-crystal X-ray diffraction. We have
collected X-ray data for several (CyNC)AuICl crystals of
both types. All of these crystals gave the same structure
reported above, and there was no correlation between the
presence of the impurity emission with the R value. The
powder from which both crystals shown in Figure 4 were
synthesized shows a uniform turquoise-green emission with
a spectrum identical to that for the nonuniform emission of
type II crystals (λmax ∼ 485 nm for both). However, we were
unable to grow single crystals that show this uniform
turquoise-green emission. Elemental analysis shows no
significant difference in the C, H, and N content for the two
crystal types and the powder. The three samples gave
satisfactory analysis for the (CyNC)AuICl formula (see the
Experimental Section). We have collected powder X-ray
diffraction from the uniformly turquoise-green-emitting
powder. The resulting powder pattern was different from that
of the calculated powder pattern for the single crystals (see
Supporting Information). This suggests the presence of
another (CyNC)AuICl species that is responsible for the
turquoise-green emission. While these data do not give the
structure of this species, it is likely either a polymorph or a
constitutional isomer, [(CyNC)2Au][AuCl2]. The experimen-
tal data suggest that the two solid forms dissociate to the
same molecule in solution. Conventional analysis methods
(1H and 13C NMR, IR, melting points, UV-vis), like
elemental analysis, did not distinguish the various forms from
one another (see the Experimental Section). Among the
various experimental techniques that we pursued, only
luminescence spectroscopy and powder X-ray diffraction
distinguished the two solid forms of (CyNC)AuICl. These
observations demonstrate the need to be very careful in
monitoring the luminescence from solid samples of such
complexes and the need to ascertain their homogeneity at
the single-crystal level.
Figure 5. Luminescence emission and excitation spectra for single crystals
of (CyNC)AuICl, (CyNC)AuIBr, and (CyNC)AuII. Excitation at 270 nm
was used to generate the emission spectra while the excitation spectra were
acquired monitoring the emission maxima.
Figure 6. Diffuse reflectance spectra for crystalline samples of (CyNC)-
AuICl, (CyNC)AuIBr, and (CyNC)AuII. The samples used were checked
first to ensure that they display only the uniform orange luminescence as
illustrated in Figure 4 for (CyNC)AuICl.
compounds are shown in Figure 6. The spectra are similar,
with each complex showing a three-band pattern in the 200-
330 nm region.
Discussion
Structural Properties and Aurophilic Interactions. The
structural data reported here show that the neutral molecules
(CyNC)AuICl, (CyNC)AuIBr, and (CyNC)AuII crystallize in
similar ways into extended, pleated chains through varying
degrees of aurophilic interactions. The molecules are orga-
nized in head-tail orientations along the chains. This
arrangement is favored by the alternating orientations of
adjacent dipoles of the molecules. The shortest Au‚‚‚Au
distances (3.3894(7) and 3.5816(7) Å) are seen in (CyNC)-
AuICl. However, it is important to note that these Au‚‚‚Au
separations are longer than those observed between the chains
of cations in [Au{C(NHMe)2}2](PF6)‚0.5(acetone) (3.1882(1)
Å)21 and in the two polymorphs of [(CyNC)2AuI](PF6)
(colorless, 3.1822(3) Å; yellow, 2.9803(6), 2.9790(6),
2.9651(6), and 2.9643(6) Å).5 The alternating Au‚‚‚Au
separations in (CyNC)AuIBr are 3.4864(9) and 3.7036(9) Å
so that the structure is best considered as containing pairs
Figure 5 shows the luminescence emission and excitation
spectra of carefully selected, homogeneously emitting crystals
of the three complexes at 77 K. The broad and structureless
emission (λmax ∼ 610 nm for (CyNC)AuICl and (CyNC)-
AuIBr and 625 nm for (CyNC)AuII) has a similar profile
for the three complexes. Interestingly, rather short-wave-
length UV excitation (λ e 280 nm) is required to generate
the orange emission in all three complexes. Lifetime
measurements using 265-nm laser excitation yielded τ ) 30,
47, and 18 µs ((<1 µs) for (CyNC)AuICl, (CyNC)AuIBr,
and (CyNC)AuII, respectively.
Diffuse reflectance spectra acquired for crystals of all three
Inorganic Chemistry, Vol. 42, No. 21, 2003 6745

White-Morris et al.
Chart 1
predictions with excellent agreement, which is amazing given
the difference in the R group (Me vs Cy) and packing effects
(dimer vs polymer). We conclude that a discussion of the
self-association of Au(I) compounds should not be limited
to aurophilic bonding as other relevant intermolecular
interactions (e.g., dipolar interactions, H-bonding) must be
considered as well.
In regard to complexes of the (RNC)AuIX class (with X
) Cl, Br, and I), there are now crystallographic data for five
series: (CyNC)AuIX (this work), (t-BuNC)AuIX,24,26,30
(MeOC(O)CH2NC)AuIX,24 (PhNC)AuIX,24 and (o-xylylNC)-
AuIX.23 Within these series, variation of the halide does not
lead to predictable changes in the relative orientations of
adjacent molecules, the mode of self-association, or the Au‚
‚‚Au separations. Only with (CyNC)AuIX are all three
complexes isostructural. With (t-BuNC)AuIX and (MeOC(O)-
CH2NC)AuIX, the chloride and bromide structures are
isostructural and the iodide structure is unique, while for
(PhNC)AuIX, the bromide and iodide structures are isos-
tructural. With (o-xylylNC)AuIX, all three complexes crys-
tallize in different space groups. The compounds (CyNC)-
AuIX (X ) Cl, Br or I), (MeNC)AuICl, (t-BuNC)AuIX (X
) Cl or Br), (PhNC)AuIBr, and (PhNC)AuII all crystallize
with the molecules arranged into zigzag chains with anti-
parallel orientations of individual molecules. The distances
between gold centers in these chains generally fall in the
range of 3.5-3.7 Å, and the aurophilic bonding interactions
within these chains are weak. In this group, (CyNC)AuICl
has the shortest contact, 3.3894(7) Å.
Optical and Photophysical Properties. The similarity in
the absorption, emission, and excitation spectra seen for this
series of isostructural complexes suggests that differences
in supramolecular organization found in the crystals do not
strongly influence the spectroscopy. Thus, the small varia-
tions seen in the diffuse reflectance spectra shown in Figure
6 for (CyNC)AuICl, (CyNC)AuIBr, and (CyNC)AuII are
consistent with the relatively small differences in the
molecular structure introduced by the differing halide groups.
A previous study of the related complex, (MeNC)AuICN,
assigned absorptions in the 200-260 nm region to metal-
ligand charge transfer (MLCT) transitions.39 A similar
assignment of the absorption bands seen in Figure 5 is
suggested for the complexes studies here.
of molecules connected by a single aurophilic interaction.
In (CyNC)AuII, the Au‚‚‚Au separations are even larger,
3.7182(11) and 3.9304(12) Å, and are beyond the range
where there is significant aurophilic interaction.
Self-association of two-coordinate gold(I) complexes can
involve an interaction through the antiparallel orientation
shown as A in Chart 1 or via the staggered orientation shown
as B in this chart. A database analysis has shown that shorter
contacts are found with the staggered orientation B.10 In the
series (CyNC)AuICl, (CyNC)AuIBr, and (CyNC)AuII, all Au‚
‚‚Au interactions are of the antiparallel type A.
The present study and several others on related complexes
offer the opportunity to gain insight into the effects of
different anionic ligands on the aurophilic bonding. Theoreti-
cal studies by Pyykko¨ and co-workers predicted an increase
in the strength of the aurophilic bonding for the H3PAuIX
system in the series of anionic ligands (X) with F < CH3 <
H < Cl < CN < Br < I < -SCH3.11a This trend has been
confirmed for the set of compounds (Me2PhP)AuIX (X )
Cl, Br, I) where the Au‚‚‚Au distances shorten as one goes
from the chloride complex (3.230(2) Å) to the bromide
complex (3.119(2) Å) and to the iodide complex (3.104(2)
Å).37 A similar trend has been seen for the pair of complexes
(1,3,5- triaza-7-phosphaadamantane)AuIX (X ) Cl, Br).38
At first sight, it appears puzzling that the trend in Au‚‚‚
Au distances observed here (with Au‚‚‚Au distances in the
order: Cl < Br < I) is the reverse of the trend observed for
the analogous complexes with phosphine ligands. However,
compounds in the isonitrile series have the antiparallel
arrangement A in Scheme 1 while compounds in the
phosphine series have the staggered arrangement B. Con-
sequently, different contributions from aurophilic interactions
and dipolar forces are present in the two different series of
complexes. A similar conclusion has been made recently by
Schmidbaur, Runeberg, and co-workers based on theoretical
predictions for the two series H3PAuIX and (MeNC)AuIX
with orientations A and B for each.30 The experimental results
for the three isostructural complexes herein support the
theoretically predicted trend of Au‚‚‚Au distances for
[(MeNC)AuICl]2 and [(MeNC)AuII]2 model dimers with the
antiparallel geometry (A), for which the calculations pre-
dicted Au‚‚‚Au distances of 3.442 and 3.792 Å, respec-
tively.30 Our experimental data verify these theoretical
All of the complexes examined here show a strikingly large
Stokes’ shift (∼21 000 cm-1 or ∼2.6 eV). A Stokes’ shift
with such a large magnitude indicates that the excited state
is rather distorted with different bonding properties from
those for the ground state.40 The lifetime data, which fall in
the range of 20-50 µs, suggest that the orange emission
occurs from a triplet excited state, as is common in
luminescent Au(I) compounds.1
Several explanations of the large Stokes’ shift present
themselves. One possibility involves exciplex formation with
(37) (a) Toronto, D. V.; Weissbart, B.; Tinti, D. S.; Balch, A. L. Inorg.
Chem. 1996, 35, 2484. (b) Weissbart, B.; Toronto, D. V.; Balch, A.
L., Tinti, D. S., Inorg. Chem. 1996, 35, 2490.
(38) Assefa, Z.; McBurnett, B. G.; Staples, R. J.; Fackler, J. P., Jr.;
Assmann, B.; Angermaier, K.; Schmidbaur, H. Inorg. Chem. 1995,
34, 75.
(39) Chastain, S. K.; Mason, W. R. Inorg. Chem. 1982, 21, 3717.
(40) (a) Omary, M. A.; Patterson, H. H. In Encyclopedia of Spectroscopy
& Spectrometry; Academic Press: London, U.K., 2000, pp 1186-
1207. (b) Lakowicz, J. R. Principles of Fluorescence Spectroscopy;
Plenum Press: New York, 1983.
6746 Inorganic Chemistry, Vol. 42, No. 21, 2003

Orange Luminescence and Structure of Isonitrile AuI
shortening of the Au‚‚‚Au distances in the excited states.
Exciplex formation has been shown to occur for a number
of solids containing [Au(CN)2]- and [Ag(CN)2]- com-
plexes.16-20 Although there is a considerable variation in the
Au‚‚‚Au distances within the series of complexes studied
here, these are ground-state distances while distances in the
luminescent excited states could be similar for the three
complexes. One should realize that short ground-state
distances are not required for the observation of exciplex
emissions, as is well documented for organic exciplexes.41
Thus, the fact that the iodo complex, for example, does not
exhibit significant ground-state aurophilic bonding does not
preclude an exciplex emission. Alternatively, the large
Stokes’ shift could originate from a geometric distortion of
the Au-C-N-C unit, which is a consistent feature in each
of the three molecules examined here. Such a distortion could
involve a localized range of bond length and bond angle
changes within the Au-C-N-C unit that result in a
significant change in the excited-state molecular structure
without displacement of the bulky cyclohexyl groups or
major alteration of the Au‚‚‚Au interactions. Since the
absorption features are consistent with an MLCT process
involving the Au-CN unit, alteration of just this portion of
the individual molecules is a reasonable consequence of this
absorption process. To judge which of these possibilities is
more likely, rigorous ab initio and density functional theory
calculations using configuration interaction methods are
being pursued to calculate the optical spectra corresponding
to transitions involving each of the aforementioned possible
excited states.
The spectroscopic data seen for the series (CyNC)AuICl,
(CyNC)AuIBr, and (CyNC)AuII contrast with recent obser-
vations on complexes of type (RNC)AuICN.36 In the latter,
variation of the alkyl groups produces solids with a range
of different patterns of self-association including simple
chains, side-by-side chains in which two strands make Au‚
‚‚Au contacts with each other, and two-dimensional sheets.
Each of these solids have distinctive absorption bands at
longer wavelengths than the molecular MLCT bands and
unique luminescence spectra with emission maxima that vary
from 371 to 430 nm. Moreover, the excitation maxima also
vary over the range of 293-353 nm. Consequently, it was
concluded that the spectroscopic features of the series are a
consequence of the supramolecular organization of molecules
within the solids and that the aurophilic interactions between
molecules were crucial for the spectroscopic absorption and
emission. The facts that the (CyNC)AuIX compounds
reported here exhibit a different mode of self-association and
that the electronic structure is different for halides from that
for cyanide make it hardly surprising to see a different
luminescence behavior for the two classes of compounds.
However, a further comparison of the properties of (CyNC)-
AuICN and (CyNC)AuICl is warranted. Although these two
complexes are not isostructural ((CyNC)AuICN crystallizes
in the monoclinic space group P21/c),36 they both form
pleated chains with significant aurophilic bonding. Both
structures involve two alternating Au‚‚‚Au distances: 3.426(3)
and 3.442(3) Å in (CyNC)AuICN and 3.3879(16) and
3.5875(16) Å in (CyNC)AuICl. However, (CyNC)AuICN
does not show the large Stokes’ shift seen for (CyNC)AuICl.
Rather, (CyNC)AuICN exhibits emissions that depend on the
excitation wavelength.36 Thus, excitation at 315 nm produces
an emission maximum at 403 nm while excitation at 360
nm produces an emission peak at 403 nm along with a
shoulder at 460 nm. Clearly, changing from a π-acceptor
anionic ligand in (CyNC)AuICN to a π-donor anionic ligand
in (CyNC)AuICl alters the electronic properties of these
complexes.
It is interesting to note that the luminescence and absorp-
tion characteristics of (CyNC)AuICl, (CyNC)AuIBr, and
(CyNC)AuII are rather similar to those communicated by
Vogler and Kunkely for the related compound (OC)AuICl,
which also exhibits a red-orange luminescence with a large
Stokes’ shift of ∼2.1 eV.32 On the basis of their rather limited
data, these authors ascribed the absorption of (OC)AuICl to
a gold d-s transition and attributed the Stokes’ shift to
aurophilic interactions between molecules in the excited state.
The crystal structure of (OC)AuICl shows that the molecules
are arranged in a head-tail antiparallel fashion (A in Scheme
1) with a short Au‚‚‚Au contact of 3.38 Å.42 Thus the
structure of (OC)AuICl resembles that of (CyNC)AuICl but
with closer Au‚‚‚Au interactions.
Experimental Section
Materials. Hydrogen tetrachloroaurate monohydrate and hydro-
gen tetrabromoaurate monohydrate were purchased from Strem
Chemicals. Cyclohexyl isonitrile, (Me2S)AuCl, and sodium iodide
were purchased from Aldrich. (THT)AuCl (THT ) tetrahy-
drothiophene) was synthesized by a modification of a published
procedure.43 All reactions were carried out under atmospheric
conditions unless otherwise indicated.
Synthesis. (CyNC)AuICl, Method 1. Tetrachloroauric acid
hydrate (340 mg, 1 mmol) was dissolved in 8 mL of water to form
a yellow solution. Cyclohexyl isonitrile (400 µL, 3.2 mmol) was
then added to the solution. A yellow solid precipitated immediately.
The solution was allowed to stir. Some of the solid dissolved, and
a brown solid formed. After 30 min of stirring, the solution was
filtered. The aqueous solution was allowed to stand for 2 h while
white flakes of a solid formed. The solid was collected by filtration
and dried under a vacuum. This solid exhibits a uniform orange
luminescence (λmax ∼ 610 nm). Colorless single crystals of the
product were grown by diffusion of n-pentane into a dichlo-
romethane solution of the complex at ambient temperature or by
cooling a nearly saturated solution of the complex in methanol at
4 °C.
(41) (a) Lowry, T. H.; Schuller-Richardson, K. Mechanism and Theory in
Organic Chemistry; Harper & Row: New York, 1981; pp 919-925.
(b) Turro, N. J. Modern Molecular Photochemistry; Benjamin/
Cummings: Menlo Park, CA, 1978; pp 135-146. (c) Lamola, A. A.
In Energy Transfer and Organic Photochemistry; Lamola, A. A.,
Turro, N. J., Eds.; Wiley-Interscience: New York, 1969; pp 54-60.
(d) The exciplex; Gordon, M., Ware, W. R., Eds.; Academic Press:
New York, 1975. (e) Kopecky, J. Organic Photochemistry: A Visual
Approach; VCH: New York, 1991; pp 38-40. (f) Michl, J.; Bonacic-
Koutecky, V. Electronic Aspects of Organic Photochemistry; Wiley:
New York, 1990; pp 274-286.
(42) Jones, P. G. Z. Naturforsch. 1982, 37b, 823.
(43) Uso´n, R.; Laguna, A. In Organometallic Syntheses King, R. B., Eisch,
J. J., Eds.; Elsevier: Amsterdam, 1986.
Inorganic Chemistry, Vol. 42, No. 21, 2003 6747

White-Morris et al.
(CyNC)AuICl, Method 2. Standard Schlenk-line techniques
under nitrogen atmosphere were followed in this method.
(THT)AuCl (560 mg, 1.64 mmol) was dissolved in 10 mL of
deaerated dichloromethane. Cyclohexyl isonitrile (220 µL, 1.80
mmol) was then added and the solution was stirred for 2 h, after
which the solvent was evaporated under reduced pressure and a
white solid formed. The solid was washed several times with
pentane, collected by filtration, and vacuum-dried. This method was
also followed using (Me2S)AuCl instead of (THT)AuCl. Thus,
reaction of 560 mg (1.90 mmol) of (Me2S)AuCl with an equimolar
or excess amount of cyclohexyl isonitrile yielded 510 mg of
(CyNC)AuICl (90% yield). Anal. Calcd for C7H11NAuCl: C, 24.61;
H, 3.25; N, 4.10. Found for this powder (which gives uniform
turquoise-green luminescence with λmax ∼ 485 nm): C, 24.71; H,
2.98; N, 4.30. Found for crystals grown from methanol (type I in
Figure 4): C, 24.89; H, 3.22; N, 4.11. Found for crystals grown
from CH2Cl2/n-pentane (type II in Figure 4): C, 24.72; H, 3.46;
N, 4.23. In addition to elemental analysis, additional techniques
that did not distinguish the two solid forms included the following
with the data presented in pairs for the samples emitting uniform
(turquoise-green, orange), respectively. Melting points: The solid
melted sharply at (127-129, 125-127) °C into a colorless liquid.
1H NMR data in DMSO: (1.36-2.10, 1.37-2.00) ppm (m, 10H)
and (4.22, 4.19) ppm (s, 1H). 13C NMR data in DMSO: (22.30,
22.27), (24.36, 24.37), (30.74, 30.75), and (54.42, 54.37) ppm. The
isonitrile carbon did not appear, which is common for isonitrile
compounds24 due to the absence of hydrogen atoms attached to
this carbon.
(CyNC)AuIBr. Cyclohexyl isonitrile (175 µL, 1.41 mmol) was
added to a dark-red solution obtained by dissolving 329.9 mg
(0.6373 mmol) of hydrogen tetrabromoaurate hydrate in 7 mL of
water in a 25-mL round-bottom flask. When the solution was stirred,
the dark-red color disappeared and the solution eventually became
clear and colorless, with a hint of a brown suspension. After 30
min of stirring, the aqueous solution was filtered into a 25-mL
round-bottom flask where it was allowed to slowly evaporate. A
white crystalline precipitate began to form immediately. The
solution was allowed to evaporate for a few days, whereupon more
crystals formed. Crystals formed in this fashion were suitable for
single-crystal X-ray diffraction. The compound was also synthesized
by metathesis of (CyNC)AuICl with NaBr following a similar
procedure to the one described below for (CyNC)AuII. Anal. Calcd
for C7H11NAuBr: C, 21.78; H, 2.87; N, 3.63. Found: C, 21.96;
H, 2.61; N, 3.54.
paratone oil on a microscope slide. Suitable crystals were mounted
on glass fibers with silicone grease and placed in the cold stream
of a Bruker SMART CCD with graphite monochromated Mo KR
radiation at 90(2) K. No decay was observed in 50 duplicate frames
at the end of each data collection. Crystal data are given in Table
1.
The structures were solved by direct methods and refined using
all data (based on F2) using the software of SHELXTL 5.1. A
semiempirical method utilizing equivalents was employed to correct
for absorption.44 Hydrogen atoms were added geometrically and
refined with a riding model.
Physical Measurements. The luminescence and diffuse reflec-
tance measurements were carried out at the University of North
Texas for crystalline material carefully examined by optical
microscopy. In some cases, we noted that the orange-emitting
crystals were coated in part with a thin overlayer of a turquoise-
green-emitting, but apparently amorphous, material. Crystals ex-
hibiting such irregularities were rejected for use in our studies.
Steady-state luminescence spectra were acquired with a PTI
QuantaMaster model QM-4 scanning spectrofluorometer equipped
with a 75-Watt xenon lamp, emission and excitation monochro-
mators, excitation correction unit, and a PMT detector. The emission
spectra were corrected for the detector wavelength-dependent
response while the excitation spectra are presented uncorrected due
to the unreliability of correction methods at short wavelengths below
250 nm, at which the samples here absorb and the xenon lamp
output is rather low. Lifetime data were acquired using a nitrogen
laser interfaced with a tunable dye laser and a frequency doubler,
as part of fluorescence and phosphorescence subsystem add-ons to
the PTI instrument. The 337.1-nm line of the N2 laser was used to
pump a freshly prepared 1 × 10-3 M solution of the organic
continuum laser dye Coumarin-540A in ethanol, the output of which
was tuned and frequency doubled to attain the 265-nm excitation
used to generate the time-resolved data. Diffuse reflectance spectra
for single crystals of the compounds packed in a 0.1-mm supracell
quartz cuvette were acquired using a 150-mm integrating sphere
interfaced to a Perkin-Elmer Lambda 900 double-beam UV/VIS/
NIR spectrophotometer. IR spectra were recorded as pressed KBr
pellets on a Matteson Galaxie Series FTIR 3000 spectrometer.
Acknowledgment. We thank the donors of the Petroleum
Research Fund, administered by the American Chemical
Society, for support of this research to A.L.B. (Grant 37056-
AC) and the Robert A. Welch Foundation (Grant B-1542)
and the University of North Texas Faculty Research Grant
for the funding of this research to M.A.O. The Bruker
SMART 1000 diffractometer was funded in part by NSF
Instrumentation Grant CHE-9808259. We thank Dr. Oliver
Chyan and Mr. Praveen Nalla for assisting with acquiring
the photographs in Figure 4 with the optical microscope and
Dr. Teresa Golden for collecting the powder X-ray diffraction
data in the Supporting Information.
(CyNC)AuII. (CyNC)AuICl (44.9 mg, 0.131 mmol) was dis-
solved in 8 mL of dichloromethane and placed in a 25-mL round-
bottom flask. Finely ground sodium iodide (107 mg, 0.714 mmol)
was added to the solution to form a suspension. This suspension
was stirred for 1.5 h in order to allow the metathesis reaction to
take place. The suspension was filtered, and the volume of the
filtrate was reduced to 3 mL under a vacuum. Petroleum ether was
added to the solution, which produced a small quantity of a white
solid. This mixture was allowed to evaporate for 2 h. Color-
less plates were formed during this process. Anal. Calcd for
C7H11NAuI: C, 19.42; H, 2.56; N, 3.23. Found: C, 19.11; H, 2.59;
N, 3.11.
Supporting Information Available: Powder diffraction data
for (CyNC)AuICl (PDF) and X-ray crystallographic files, in CIF
format, for (CyNC)AuICl, (CyNC)AuIBr, and (CyNC)AuII. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Infrared Data. (CyNC)AuICl: 2933, 2856, 2256, 1448, 1361,
1043, and 889 cm-1. (CyNC)AuIBr: 2933, 2856, 2254, 1448, 1363,
1043, and 896 cm-1. (CyNC)AuII: 2933, 2854, 2248, 1444, 1360,
1041, and 889 cm-1
.
IC0341946
X-ray Crystallography and Data Collection. The crystals were
removed from the glass tubes in which they were grown together
with a small amount of mother liquor and immediately coated with
(44) SADABS 2.0, Sheldrick, G. M. based on a method of Blessing, R. H.
Acta Crystallogr., Sect. A 1995, A51, 33.
6748 Inorganic Chemistry, Vol. 42, No. 21, 2003